This invention relates to a process for separating 2,6-xylenol from a mixture of cresylic acids containing 2,6-xylenol; cresol isomers, o-, m-, and p-cresol; and higher homologs.
Cresylic acids are naturally-occurring phenolic compounds found in coal tar distillates derived from low-temperature carbonization of bituminous coal. These compounds also occur in petroleum, in coke oven tars, in oil shale distillates, in coal hydrogenation oils and the like. The 2,6-xylenol appears in certain reaction products as well as in naturally-occurring cresylic acid mixtures. One example is the catalytic methylation of either phenol or o-cresol which provides mixtures containing 2,6-xylenol. The 2,6-xylenol (2,6-dimethyl-phenol) is a valuable material for use in antioxidants, plasticizers and the like. It also may be useful as a monomer in the preparation of polymers.
The problem of separating 2,6-xylenol in good yield and high purity from cresylic acids is difficult because the cresol isomers have boiling points that are similar to the boiling point of 2,6-xylenol. According to Bulletin 606, Bureau of Mines, "Properties of Compounds in Coal-Carbonization Products" by H. C. Anderson and W. R. K. Wu, the following cresylic acids at a pressure of 760 mm of mercury have the following boiling points:
______________________________________ 2,6-xylenol 201.degree. C. o-cresol 191.003.degree. C. m-cresol 202.231.degree. C. p-cresol 201.940.degree. C. ______________________________________
In view of the relatively close boiling points of these cresylic acids, the prior art has resorted to both physical and chemical methods for separating such mixtures. Successive fractional distillation, fractional azeotropic distillation, double solvent extraction, azeotropic distillation with a non-polar branched alkene and other methods have been suggested. The separation by the process of this invention is simpler and more economical than the prior art processes. The process involves formation of arylphosphates, especially triarylphosphates, primarily from the cresylic acids.
The preparation of alkylated triaryl phosphates have been known for many years. The classical method of forming triarylphosphates is by the reaction of alkyl phenol with phosphorus oxychloride. In British Pat. No. 1,146,173 by Wilfred Pickles and Donald Richard Randall entitled, "Production of Triaryl Phosphate," a method of triarylphosphate production using a catalyst is disclosed. This patent also discloses in its comparative examples that phosphorylation of commercially available cresols produces an inferior phosphate ester than the alkylated phenols of its process. The commercially available cresol contained around 1.5% 2,6-xylenol along with o-cresol, 2,4-xylenol, 2,3-xylenol, 2,5-xylenol, 3,4-xylenol, 3,5-xylenol and predominantly p-cresol and m-cresol. This disclosure shows that triarylphosphates can be formed from the above compounds.
An article entitled, "Structural Effects in Reactions of Organophosphorus Compounds. I. Reactions of Phosphorus Oxychloride with Hindered Phenols," discusses the reaction of phosphorus oxychloride (POCl) with 2,6-dialkyl-phenols to form triaryl phosphates. The conclusion of the article is that 2,6-dialkyl phenols exhibit little steric hindrance on reaction with POCl.sub.3 unless the alkyl substituent is bulky like a t-butyl alkyl substituent. Where these bulky substituents are present, reaction with POCl.sub.3 occurs only in the presence of Friedel-Crafts type catalysts after either a dealkylation or a rearrangement has occurred. With smaller alkyl substituents the phenols show a relatively slower rate of reaction with phosphoryl chloride. In this article, 2,6-xylenol was reacted directly with POCl.sub.3 and other phosphorochloride compounds with and without a catalyst. Several reactions were completed while other reactions did not start. When an equimolar amount of catalyst was used, the reaction went to completion and formed 2,6-dimethylphenylphosphorodichloridate.
It is an object of this invention to provide a process for separating 2,6-xylenol from a mixture of cresylic acids which will permit recovery of the 2,6-xylenol in good yields at a high purity.